Recent studies have revealed that NO, which is at least one of the endothelium-derived relaxing factors (EDRF) (Nature, 327, 524-526, 1987; Nature, 333, 664-666, 1988), not only has a vasodilation effect and a platelet aggregation and adhesion inhibiting effect but also plays an important role in the maintenance of biological functions as an intra-/inter-cellular signaling molecule. (Trends Pharmacol. 10, 428-431, 1989; Trends Neurosci. 14, 29-39, 1987; Trends Biochem. 16, 81-83, 1991; Trends Neurosci. 13, 1-6, 1992; Trends Pharmacol. 12, 130-131, 1991; Trends Pharmacol. 12, 87-88, 1991; Trends Pharmacol. 12, 125-128, 1991).
It has also been established that NO is produced not only from endothelial cells but also from other cells such as neurons, glial cells and macrophages and at least three distinct clones encoding independent NOS enzymes have been isolated (Nature, 351, 714-718, 1991; Proc. Natl. Acad. Sci. USA, 89, 11141-11145, 1992; Proc. Natl. Acad. Sci. USA, 89, 6348-6352, 1992; FEBS Lett., 307, 287-293, 1992; J. Clin. Inv., 90, 2092-2096, 1992; Science, 256, 225-228, 1992; J. Biol. Chem., 267, 6370-6374, 1992; Proc. Natl. Acad. Sci. USA, 90, 11419-11423, 1993; Biochem., 32, 11600-11605, 1993; Proc. Natl. Acad. Sci. USA, 90, 3491-3495, 1993; FEBS Lett., 316, 175-180, 1993; Biochem. Biophys. Res. Commun., 191, 89-94, 1993; Proc. Natl. Acad. Sci. USA, 89, 6711-6715, 1992; Biochem. Biophys. Res. Commun., 191, 767-774, 1993; Proc. Natl. Acad. Sci. USA, 90, 9730-9734, 1993).
It is also interesting to note that the pathophysiological role of NO has been unravelled at an accelerated rate (Pharmacol. Rev., 43, 109-142, 1991; Neurol. Prog., 32, 197-311, 1992; FASEB J., 6, 3051-3064, 1992).
Among the NOS inhibitors, N.sup.G -nitro-L-arginine (L-NNA) has been found to be capable of ameliorating ischemic cerebral infarction and edema (Eur. J. Pharmacol. 204, 339-340, 1991; Neurosci. Lett., 147, 159-162, 1992). This finding opened a road to the application of a substance having a NOS inhibiting activity to a therapeutic of cerebrovascular diseases.
In the brain region where the blood flow is interrupted, cytotoxic edema occurs first, followed by vasogenic edema. Brain edema is developed several hours after the occurrence of cerebral ischemia and its progress continues for about one week from the onset in clinical settings. Thereafter, the brain edema decreases gradually and, depending on the focal range of infarction, the edema persists as an infarcted area from one to three months. Since the brain is covered with the rigid skull, cerebral edema causes an increase in the brain volume. If the cerebral edema exceeds a certain limit, there occurs an abrupt increase in the tissue pressure and the intracranical pressure, often inducing fatal hernia and eventually aggravating the encephalopathy to determine the scope of the subsequent infarcted area (J. Neurosurg. 77, 169-184, 1992). Thus, the treatment of cerebral edema which is critical to the patient's life and the prognosis of his disease is clinically a very important objective. The three primary methods currently used to treat cerebral edema are hyperpnea, the drainage of cerebrospinal fluid and the use of hypertonic solutions, steroids or the like; however, in almost all cases, these methods provide only temporary ameliorative effect and there is not much promise for the therapeutic efficacy to be finally achieved (J. Neurosurg. 77, 337-354, 1992). Therefore, it has been desirable to develop drugs that have an entirely different mechanism of action and which will prove effective in the treatment of ischemic cerebrovascular diseases.
The present inventors previously found that L-NNA had a capability for ameliorating cerebral edema and infarction that were developed after focal cerebral ischemia (Neurosci. Lett., 147, 159-162, 1992), as well as the neuronal cell death that was developed after global cerebral ischemia (Eur. J. Pharmacol. Env. Tox., 248, 325-328, 1993). However, it has also been reported that relatively high doses of NOS inhibitors are not only ineffective against ischemic brain damage but also they sometimes aggravate the situation (J. Cereb. Blood Flow Metab., 14, 175-192, 1994).
A presently dominant theory based on DNA analyses holds that NOS exists in at least three isoforms, namely, neuronal constitutive NOS (N-cNOS) which is present constitutively in neurons, endothelial constitutive NOS (E-cNOS) which is present constitutively in endothelial cells, and inducible NOS (iNOS) which is expressed on stimulation by cytokines and lipopolysaccharides in macrophages and many other cells. Among these three isoforms, N-cNOS and E-cNOS are calcium-dependent whereas iNOS is calcium-independent (FASEB J. 16, 3051-3064, 1992).
It has become evident that during and shortly after cerebral ischemia there is an excessive accumulation of excitatory amino acids in the extracellular space, and subsequently an increased level of intracellular Ca.sup.2+, predominantly by way of the N-methyl-D-aspartate (NMDA) class of glutamate receptor activation. These cellular events have a crucial triggering role in the elaboration of ischemic brain damage within the neurons. (J. Cereb. Blood Flow Metab. 1, 155-185, 1981; J. Neurosurg. 60, 883-908, 1984; Trends Neurosci. 11, 465-469, 1988; J. Cereb. Blood Flow Metab. 9, 127-140, 1989). As already mentioned, N-cNOS is calcium-dependent, so inhibiting the hyperactivity of this type of NOS isoform by a particular NOS inhibitor would exhibit neuroprotective effect (Annals Neurol. 32, 297-311, 1992).
Since NO is at least one of EDRF, if E-cNOS is inhibited to less than a certain level, the cerebral blood flow will decrease whereas the blood pressure will increase, thereby aggravating the dynamics of cerebral micro-circulation, possibly leading to an expansion of the ischemic lesion. Therefore, if a therapeutic for ischemic cerebral diseases is to be developed, a NOS inhibitor is desirable that does not inhibit E-cNOS as effectively as N-cNOS (Eur. J. Pharmacol., 204, 339-340, 1991; Proc. Natl. Acad. Sci. USA, 88, 6368-6371, 1991).
Recently and after the date of filing of a product patent application by the present inventors (Feb. 25, 1994), it was reported that .delta.-(S-methylisothioureido)-L-norvaline, or the claimed compound of the present invention, was capable of inhibiting both N-cNOS and iNOS in a dose-dependent manner J. Med. Chem. 37, 885-887, 1994 (issue date: Apr. 1, 1994); Advance ACS Abstracts (J. Med. Chem. In abridged form; issue date: Mar. 1, 1994)!. The paper sets forth percent inhibitions of iNOS and cNOS which is equivalent to N-cNOS. However, the paper says nothing about the action on E-cNOS, nor does it make any mention of the potential use of .delta.-(S-methylisothioureido)-L-norvaline as a therapeutic for cerebrovascular diseases.